1. Field of the Invention
The disclosed technology relates to methods for blister-free passivation of a silicon surface, which may be used for surface passivation of silicon photovoltaic cells.
2. Description of the Related Technology
The silicon photovoltaic industry uses ever thinner wafers to significantly reduce the silicon content per wafer, thereby reducing the cost of photovoltaic cells. Consequently the surface-to-volume ratio of the cells increases, and therefore the need for providing a good surface passivation of bulk silicon photovoltaic cells gains importance.
Because of the high sensitivity of the silicon bulk lifetime to high temperature processes, especially for multi-crystalline silicon wafers, low-temperature surface passivation processes are being developed for future industrial high-efficiency silicon photovoltaic cells. For example, it has been shown that thin films of aluminum oxide (Al2O3) grown by atomic layer deposition (ALD) can provide a good surface passivation on p-type and n-type silicon wafers. On p-type crystalline silicon surfaces, a fixed negative charge density within the Al2O3 layer can induce an accumulation layer that provides an effective field-effect passivation. Therefore, ALD-deposited Al2O3 can advantageously be used for p-type rear surface passivation of photovoltaic cells, such as for example for PERC-type (passivated emitter and rear contacts) photovoltaic cells and for PERL-type (passivated emitter rear locally diffused) photovoltaic cells. On top of the ALD-deposited Al2O3 layer an additional dielectric layer, such as a silicon nitride layer or a silicon oxide layer can be provided.
In photovoltaic cell fabrication processes, after depositing the Al2O3 passivation layer, often an annealing at high temperature is performed, such as for example a metal firing. For example, in a fabrication process for PERC-type silicon photovoltaic cells, a co-firing process at a temperature above 835° C. is typically done. When using at the rear side of such a cell an Al2O3 passivation layer or a stack comprising an Al2O3 layer and a dielectric layer such as a silicon nitride layer, it is known that such high temperature annealing processes often lead to blistering of the passivation layer, i.e. partial delamination of the Al2O3 layer and formation of bubbles or blisters at the interface between the passivation layer and the silicon surface. Such blisters have a negative effect on the passivation quality of the passivation layer, leading to a reduced open-circuit-voltage Voc and thus a reduced energy conversion efficiency of the photovoltaic cell.
For example, in “Firing stable Al2O3/SiNx layer stack passivation for the front side boron emitter of n-type silicon solar cells”, 25th EPVSEC, September 2010, Valencia, Spain, A. Richter et al report blistering effects in an Al2O3/SiNx stack after firing at high temperature (750° C. to 850° C.). It was found that blistering is significantly affected by the Al2O3 layer thickness and deposition temperature. More in particular, stronger blistering is observed with an increasing layer thickness and for a decreasing deposition temperature.